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things you should not put into your fish tank

20 Things you Should Never Put in your Fish Tank

As a fish mum or dad, you want to give your fish the best life possible.

One of the most important things that you can do is make sure that they have a clean and healthy environment.

There are some obvious items that should never be put into a fish tank. Bleach, non aquarium safe chemicals, and any sharp objects are all things that apply to this category.

But there are other items that people may not be aware of, for example, ceramics, plastic toys and seashells, that can harm your fish if put into their tank.

Here is a list of 20 things that should never be put into a freshwater fish tank. These are the more obvious items. I will go over a few of the ridiculous ones later on in the article!

Plastic Toys.

Plastic toys is actually a bit of a grey area.

You can get plastic toys for your tank from pet shops. These are aquarium safe, so you don’t need to worry about them. They will have been treated and sealed beforehand, or they are a more inert plastic. The more inert a plastic, the less chance of it leaking chemicals.

Children’s plastic toys are a different story.

If a toy has stickers on, is painted or glued in any way, or has material on it, then it cannot go into a fish tank.

Similarly, if you are debating whether to put your child’s Action Man in, I would think again, as anything with joints can also prove a danger to fish as they could get caught in them.

I am sorry if this has ruined your plans for an Avengers meets Godzilla aquarium mash up but the whole point of an aquarium is to have living fish in them!

However, there is one type of plastic toy that seems to be ok to add into your tank – Lego.

Now, if you go to the Lego official website, they do say that they don’t recommend putting their toys into an aquarium.

It seems to be a bit of ‘do what we say, and not as we do’ talk going on though.

This is Atlantis at Legoland, Gunzburg, Germany…

Atlantis at Legoland
Do as we say, and not as we do..

There are hundreds, if not thousands of examples of Lego Aquarium set ups on Pinterest and on YouTube.

Lego plastic is a very hard plastic which means it shouldn’t leach in your tank.


Seaweed in it’s natural form is not really suitable to put in your fish tank because it thrives in saltwater or brackish water.

While it is in no way dangerous for the fish, if put into the tank in plant form your fish will all want to eat it rather than live with it. There is also the danger that once it starts to break up, it can clog your filter up.

You can get seaweed sheets for fish which they absolutely love (they smell awful though, so be warned).

These seaweed sheets are perfect for fish. Nori sheets are ideal as they are totally natural.

Your fish will thank you for them!

Plants that are not Aquarium Safe.

Unfortunately, you can’t pick a nice plant from your garden and add it into your tank.

There are 2 main types of plants-terrestrial and aquatic plants.

A terrestrial plant is a plant that grows on land. An aquatic plant is a plant that grows in water. Pretty self explanatory really!

Aquatic plants have evolved in water and do not need soil or dirt for their roots like terrestrial plants.

Aquatic plants also require less light because they can rely on the natural sunlight that penetrates through water.

There are a few reasons why you shouldn’t put terrestrial plants into an aquarium.

One reason is that many aquatic plants require less light and won’t be able to grow in the limited artificial light provided by a fish tank, meaning they will need constantly changing conditions if they’re expected to thrive.

Another problem with putting terrestrial plants into an aquarium is that they require soil or dirt for their roots. This could ruin the filtration system of a fish tank because as these plants grow, they will release large amounts of nutrients into the water and cause algae to flourish.


Many people are under the impression that coral is a plant or a rock.

It is, in fact, an animal.

Definition of coral:

substance like rockformed in the sea by groups of particular types of small animal
Cambridge English Dictionary

Putting coral into your freshwater tank will mess your water parameters up as it will release calcium into your tank, which will affect the hardness and pH levels of your water.

Water that is Too Cold or Too Hot.

This is a fairly obvious thing, even to a beginner. Tropical fish are used to warmer water. If your water is not within the recommended parameters for the fish you have in your tank, they will start to struggle, and will eventually die.

The temperature needs to be constant as well, which is why 2 of the most important pieces of kit you need is an aquarium heater, and a thermometer.

Some fish are used to quite different temperatures as well, so if you are building a community tank, make sure the fish you choose all like the same sort of temperature.

Anything with Chemicals in it.

Cleaning products like detergents or bleach should never be added to a live aquarium. These are toxic to fish and will kill them.

The only exceptions are for any aquarium specific items that contain chemicals. These can be bought from your local aquarium and you must follow the instructions to the letter.


seashells on the beach
Seashells on the sea shore

There are 2 main reasons why you should not put seashells into a freshwater aquarium.

The first, is, like coral, it will release calcium into your water. This is something you do not want to happen.

Secondly, the edges of the shells tend to be quite sharp, so is therefore a danger to any little fish swimming close to it.

Alcohol of any Kind.

I would sincerely hope that people would not be tempted to put any form of alcoholic drink into a fish tank.

What I am referring to more here, however, are items like alcohol based wipes and cleaning products.

Even if you were to rinse everything off afterwards, the alcohol could still be left behind on a surface somewhere in the tank, and it will kill your fish.

Too Much, or the Wrong Kind of Food.

overfeeding fish
Today’s menu…

While many people correctly think that feeding their fish is a very good idea, overfeeding can cause major problems in an aquarium because you’re adding to much food into one place at once. Most of this food won’t even be eaten by your fish and it could start rotting or decomposing before your fish even has a chance to eat it. This will then lead to ammonia spikes if not removed.

While researching this article, I was amazed at some of the foods mentioned when people enquire if they are ok for fish!

Some of the items I have seen mentioned are:

Bread (no)

Bacon (no)

Cat Food (no)

Hot Dogs (NO!)

Chocolate Milk (I’m not even answering this one!!)

While there are some forms of human food that fish can have as a little treat every so often, they are not dogs. You don’t add your leftover roast dinner into the tank for them to finish off!

Fish are fragile little things. They need to have a suitable diet to keep them healthy, happy, and alive!

Any Chemicals that are Not Intended for Aquarium Use.

things like bug spray, toilet bowl cleaner and other cleaners can kill your fish. They also have harmful fumes which could bleach the tank or ruin any decorations in there.


Metals are heavy and could disrupt the tank’s delicate balance of water, so it is best to avoid putting any metal objects into an aquarium. They can also corrode in the water, which would be very dangerous.

Rocks that are Not Aquarium Safe.

You would be best taking advice from your local aquarium with regards to rocks. You can’t really add any old rock to your tank. Rocks can have sharp edges or can be too heavy which can harm your fish if they move.

Some rock compositions can also alter the hardness or pH of your tank water.

Dirty Hands.

It’s not so much dirty hands, but hands that haven’t been cleaned in water before putting them into the tank.

It is probably not so surprising just how many different things our hands will come into contact with during the course of the day.

If you need to go into your fish tank, you must always remember to wash your hands in water first, so you don’t contaminate the tank. Don’t use soap, as this will harm the fish.

Untreated Wood.

Some types of wood are definitely not suitable for a fish tank, as they are a soft wood, so will start to break down after a while.

Other types of harder wood are ok in an aquarium but it is much safer to get this wood from a reputable supplier of aquarium products. This way, you will know that the wood has been cured beforehand to make it safe.

Sand from a Beach.

You should never add sand from a beach into your aquarium, and especially to a freshwater aquarium.

Beach sand is more than likely to be contaminated due to many different factors, bacterias, parasites, pollution, high salt levels etc.

You can buy aquarium safe sand online and in pet shops which don’t contain any toxins.

This extra fine aquarium sand is really popular.

It has excellent reviews on Amazon and has many repeat customers.


Anything glazed, which most ceramics are, should not be put into a fish tank as the glaze will start to dissolve which will be toxic to the fish.

If they are marked as ‘dinnerware safe’ then they should be ok to put in a tank, although, I don’t really understand why anyone would want to add a dinner plate to an aquarium?

Anything Painted.

There are one or two types of paint that are safe to go in an aquarium; there a few spray paints that are specifically for reef tanks. Everything else really should be avoided.


what is a paludarium
Example of a Paludarium

Some people mistakenly think it is ok to add a few reptiles or turtles into a fish tank with the fish. It can be done in specific set ups, known as a paludarium. A paludarium is a bit of a snazzy set up which consists of terrestrial plants and creatures, alongside aquatic ones. The tanks need to be on the very large side, as the water is only partway up the tank, like in the image above.

If you were to just add a lizard or a terrapin to your bog standard aquarium, your fish will be seen as lunch by the newcomers.

Dry Ice.

This is a fish tank. It’s not a rock concert or a scene from Flash Gordon.

There is no need to even think about wanting to put dry ice in your fish tank.

But, just in case you’ve momentarily lost your mind and are thinking about whether it is safe to do so-it’s not, because it will release carbon dioxide, which will suffocate your fish and also cause your aquarium to explode.

Don’t be that person, please.

Anything Sharp.

This should be another one of those common sense answers, but people do still ask if you can put anything sharp into a fish tank.

Of course you can’t!

It’s not just sharp corners you need to look out for. You also need to check for any sharp edges.

I always make sure I run my hand over any edges and corners on anything I am wanting to add to the tank, especially ornaments, to make sure everything is smooth.

Related Posts:

The best ornaments to add to your tank.

Where to buy tropical fish online safely.

Will a fish tank cycle on its own?

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